1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a pattern mask, a liquid crystal injection process and a method of fabricating an LCD device using the same.
2. Discussion of the Related Art
In general, an LCD device includes an array substrate where thin film transistors are arranged in a matrix form, a color filter substrate where color filters are formed, and a liquid crystal layer interposed between the array substrate and the color filter substrate.
A liquid crystal (LC) cell is fabricated by aligning the array substrate with the color filter substrate such that a common electrode of the color filter substrate faces a pixel electrode of the array substrate, a liquid crystal material is then injected into a gap between the array substrate and the color filter substrate, an injection hole is sealed, and then polarizers are attached on the outer sides of the two substrates. Light transmittance of the LC cell is controlled by a voltage applied to the pixel and common electrodes, and an image is displayed by a light shutter effect.
The LC cell forming process includes forming alignment layers and an LC cell gap, and cutting cells, while an array substrate forming process and a color filter substrate forming process include repeated processes.
FIG. 1 is a flow chart showing a process of fabricating an LC cell according to the related art. In FIG. 1, a step S1 includes preparation of an array substrate having thin film transistors and pixel electrodes, and a color filter substrate having a color filter layer and a common electrode.
Next, a step S2 includes formation of first and second alignment layers on the pixel electrode and the common electrode, respectively. The formation of the first and second alignment layers includes coating thin polymer films, such as polyimide, on the pixel and common electrodes, and rubbing the thin polymer films in a predetermined direction using a rubbing fabric. The thin polymer films must have a uniform thickness, and the rubbing process must be performed uniformly along the thin polymer films. Accordingly, an initial orientation of liquid crystal molecules is determined by the rubbing process. The liquid crystal molecules are moved by the rubbing to display uniform images.
Then, a step S3 includes formation of a seal pattern on either the array substrate or the color filter substrate, thereby providing a cell gap for accommodating a liquid crystal material between the array substrate and color filter substrate. In addition, the seal pattern prevents the injected liquid crystal material from leaking through the seal pattern. The seal pattern is commonly formed by using a screen-printing method or a dispensing method that includes a mixture of thermosetting resin and glass fiber.
During a step S4, a plurality of spacers are sprayed on one of the array substrate and color filter substrate using a spacer spraying method to maintain a uniform gap between the array substrate and the color filter substrate. The spacer spray method can be divided into two different types, such as a wet spray method that includes spraying a mixture of alcohol and spacer material, and a dry spray method that includes spraying spacer material alone. Here, the seal pattern and the spacers are formed on different substrates. For example, the seal pattern may be formed on the color filter substrate that has a flat surface, and the spacers may be formed on the array substrate.
At a step S5, the array substrate and color filter substrate are aligned and are attached to each other along the seal pattern. An alignment accuracy of the substrates is determined by an alignment margin, wherein the alignment accuracy of several micrometers is required since light leakage occurs if the substrates are misaligned beyond the alignment margin.
At a step S6, the attached substrates are divided into unit cells using a cell cutting process because a plurality of liquid crystal cells are formed on a large substrate. The cell cutting process includes a scribing process that forms scribe lines on a surface of the substrate using a diamond pen or a cutting wheel of tungsten carbide, wherein a hardness of the diamond pen or cutting wheel is higher than a hardness of the substrate, which is formed of glass. Then, a breaking process is performed to divide the unit cells by applying a force.
AT a step S7, a liquid crystal material is injected into each individual unit cells between two substrates using a vacuum injection method, wherein each unit cell has an area of several square centimeters and a cell gap of several micrometers. The vacuum injection process employs a pressure difference between an interior of the unit cell and an exterior of the unit cell.
FIG. 2 is a cross-sectional view schematically illustrating a liquid crystal injection process using a vacuum injection method according to the related art, and FIG. 3 is a flow chart showing the liquid crystal injection process of FIG. 3. As shown in FIG. 2, in order to inject a liquid crystal material 10 into a liquid crystal cell 2, a vacuum chamber 6 where the liquid crystal cell 2 is placed therein and a container 8 is filled with the liquid crystal material 10.
First, at a step S10 of FIG. 3, the liquid crystal cell 2 is disposed in the vacuum chamber 6 where the liquid crystal material 10 is to be injected into the liquid crystal cell 2, and then air in the liquid crystal cell 2 is removed. On the other hand, the container 8 is disposed in a bubble removing room, and then air in the liquid crystal material 10 within the container 8 is removed. At this time, the vacuum chamber 6 has an inner pressure of about 10−6 Torr and the bubble removing room has an inner pressure of about 10−3 Torr.
If fine bubbles in the liquid crystal material 10 are injected into the liquid crystal cell 2, the fine bubbles are combined to form large foams as time passes. Thus, a bubble removing process should be performed. That is, the liquid crystal material 10 is left under a vacuum condition for a long time to remove the fine bubbles in the liquid crystal material 10. The bubble process may be carried out by simultaneously loading the liquid crystal material 10 while the inside of the liquid crystal cell 2 is under vacuum.
A dipping method that the unit liquid crystal cell 2 is soaked in a liquid crystal tray has been widely used, but consumes a large amount of the liquid crystal material 10. For this reason, a touch method that only an injection hole 4 touches the liquid crystal material 10 may be used. Hereinafter, the touch method will be explained.
After the air in the liquid crystal cell 2 and the fine bubbles in the liquid crystal material 10 are sufficiently removed, at a step S20, the container 8 including the liquid crystal material 10 is located in the vacuum chamber 6 as shown in FIG. 2, and the injection hole 4 of the liquid crystal cell 2 is dipped into the container 8. Since there is no difference in pressure between the liquid crystal material 10 and the inside of the liquid crystal cell 2, the liquid crystal material 10 is injected into the inside of the liquid crystal cell 2 due to the capillary phenomenon at the initial stage of the liquid crystal injection.
After that, at a step S30, a slow vent process that a nitrogen gas (N2) is injected into the vacuum chamber 6 is performed to maintain the inside of the vacuum chamber 6 under the atmospheric pressure. Then, the liquid crystal material 10 is injected into the inside of the liquid crystal cell 2 due to the difference in pressure between the liquid crystal material 10 and the inside of the liquid crystal cell 2.
At a step S40, when the inside of the liquid crystal cell 2 is perfectly filled with the liquid crystal material 10, the liquid crystal cell 2 is left under the atmospheric pressure for about 2 hours.
Accordingly, the liquid crystal material 10 is injected in the liquid crystal cell 2 through the above-described processes. Injection time of the liquid crystal material 10 depends on a size of the liquid crystal cell 2. A 10.4 inch liquid crystal cell requires about 6 hours 50 minutes, a 12.1 inch liquid crystal cell requires about 8 hours, and a 14.1 inch liquid crystal cell requires about 10 hours. After the vacuum injection process is completed, the injection hole 4 is sealed to prevent leakage of the liquid crystal material 10 out of the liquid crystal cell 2. In general, an ultraviolet (UV) curable resin is injected into the injection hole using a dispenser, and ultraviolet light is irradiated onto the resin to harden the resin and seal the injection hole 4.
However, the vacuum injection process is time-consuming and wastes a large amount of the liquid crystal material. In addition, problems in the liquid crystal cell may be caused because it is difficult to control an amount of the injected liquid crystal material during the injection process. Moreover, an accompanying apparatus such as the vacuum chamber should be fully equipped.